1. Field of the Invention
This invention, in general, relates to heterocyclic carbon compounds including a carbocyclic nucleus fused to a 6-membered ring containing one heterooxygen atom and, in particular, to a process for converting acylated flavanones to 3-deoxyanthocyanidins.
2. Description of the Prior Art
Anthocyanins are a family of plant pigments widely occurring in nature which are responsible for the yellow, red and purple hues of most flowers and fruits. The term "anthocyanin" chemically designates a substituted flavylium (or 2-phenylbenzo-pyrylium) salt, the cation of which has the structure: ##STR1## the substitutents at the 3-8 and 2'-6' positions in the case of known naturally occurring anthocyanins denoting either H, OH, OCH.sub.3 or O-sugars. The latter, corresponding to monosides, biosides and triosides usually containing glucose, galactose, rhamnose, arabinose or xylose, are found almost exclusively at the 3 and 5 positions.
The term "anthocyanidin" refers to the aglycone of any anthocyanin, i.e., the substituted flavylium salt freed of all sugar components.
The color of anthocyanidins is dominated by the nature of the substituent at the 3 position. When it is H, the resulting 3-deoxyanthocyanidins have .lambda..sub.max .about.470 nm in water at pH 4 or lower, and give yellow solutions. The presence of an oxygenated substituent at the 3 position (OH, OCH.sub.3, O-sugar) shifts the absorption spectra bathochromically, giving rise to red anthocyanidins (.lambda..sub.max .about.530 nm). P. Stevenson, J. Molecular Spectroscopy, 18, 51-58 (1965).
These pigments have been associated with the human food chain since the dawn of civilization, being consumed as part of fruits, grains and vegetables. The absence of scientific records ascribing ill-effects to the consumption of anthocyanidins as part of the regular diet makes them attractive additives for food coloration. L. Jurd, U.S. Pat. No. 3,266,903 (Aug. 16, 1966); U.S. Pat. No. 3,301,683 (Jan. 31, 1967) and U.S. Pat. No. 3,314,975 (Apr. 18, 1967); and C. F. Timberlake, Ger. Pat. No. 1,904,810 (Oct. 2, 1969).
Of the anthocyanidins, the present invention is concerned with 3-deoxyanthocyanidins and their preparation from acylated flavanones, i.e., those containing the functional group ##STR2## hereinafter referred to as OCOR, especially those derived from naturally occurring flavanones. Synthesis of 3-deoxyanthocyanidins has been accomplished in the laboratory by acid-catalyzed condensation of substituted o-hydroxybenzaldehydes with appropriate acetophenones. G. M. Robinson, R. Robinson and A. R. Todd, J. Chem. Soc., 809(1934), and previous papers in this series. This method is exemplified below by the synthesis of apigeninidin, a constituent of certain varieties of corn and sorghum. E. D. Styles and O. Ceska, Phytochem. 14,413 (1975) and W. K. Nip and E. E. Burns, Cereal Chem. 48, 74 (1971): ##STR3##
Although this method is useful in the laboratory, its industrial practice could be constrained by the availability and cost of raw materials, particularly in connection with the synthesis of naturally-occurring anthocyanidins having extensive phenolic substitution.